Certain types of intermediate frequency (IF) receivers, such as a zero-IF type receiver, generally include a radio frequency (RF) converter having a local oscillator (LO) frequency that is near the center of the RF input bandwidth. For such a receiver, because the LO and RF frequencies may be equal or nearly equal, it is important that the LO signal leakage out of the input of the receiver be small. However, this is difficult to achieve because the LO signal generally cannot be separated from the RF signal by filtering.
At the RF converter, the LO signal typically has a significant amplitude, and mixers usually have limited isolation from the LO port to other ports, e.g., RF and IF ports. Isolation in the mixer can be maximized by applying direct current (DC) offset current into the IF port of the mixer, and a leakage detector can be used to measure the leakage signal as part of a feedback control loop which minimizes the leakage amplitude.
To date, various types of circuits have been developed to maintain low amplitudes of LO leakage from the RF input. While each idea may have some benefits, there are also a number of associated problems. Generally, there is a detector on the RF port side of the mixer to measure leakage LO power, and the detected amplitude may be used to form correction signals through a feedback control loop.
FIG. 1 illustrates an example of a presently-used implementation 100 of a LO leakage detector. Indeed, the illustrated circuitry represents a particularly popular implementation of a leakage detector. In the example, there are two input ports (an RF input port 102 and a LO input port 104) and a mixer 112 for providing an output signal at an output signal port 114. A directional coupler 106 in the RF path transfers a portion of the reverse-travelling LO leakage signal (FLO) to a wideband detector 108. The detected output is measured by way of an analog-to-digital converter (ADC) 110.
While the implementation illustrated by FIG. 1 has certain advantages (e.g., the leakage LO signal is usually detected and the feedback control may remain engaged), the LO leakage signal is generally lost in thermal noise of the wideband detector 108 output, and correction does not typically work to certain low amplitudes.
Accordingly, there remains a need for improved signal detection circuits and devices having an LO input.